Lignocellulosic biomass represents one of the most abundant renewable resources on Earth. It is formed of three major components—cellulose, hemicellulose, and lignin—and includes, for example, agricultural and forestry residues, municipal solid waste (MSW), fiber resulting from grain operations, waste cellulosic products (e.g., paper and pulp operations), and energy crops. The cellulosic and hemicellulosic polymers of biomass can be hydrolyzed into their component sugars, such as glucose and xylose, which can then be fermented by microorganisms to produce ethanol. Conversion of even a small portion of the available biomass into ethanol could substantially reduce current gasoline consumption and dependence on petroleum.
Multiple conversion processes have evolved for breakdown of biomass to produce bioenergy. These processes vary from multi-enzyme and multi-fermentation approaches called separate hydrolysis and fermentation (SHF) [Wilke et al. (1976) Biotechnol. Bioeng. Symp. 6:55] to simpler simultaneous cellulose hydrolysis (or saccharification) and fermentation (SSF) [Takagi et al. (1977) in Proceedings of the Bioconversion Symposium, Indian Institute of Technology, New Delhi, pp. 55-571; Spindler (1988) Appl. Biochem. Biotechnol. 17:279-294; Alfani (2000) J. Ind. Microbiol. Biotechnol. 25:184-192]. In an SHF process, the cellulosic biomass is hydrolyzed with cellulases to liberate fermentable glucose followed by a separate step for fermentation to ethanol. The SSF process combines the enzymatic hydrolysis and fermentation simultaneously, reducing the process complexity. A natural extension is simultaneous saccharification and cofermentation (SSCF) using microorganisms that are able to convert both hexose and pentose sugars to ethanol. This process simplification culminates with the development of fermentation microorganisms that produces their own enzymes for cellulose hydrolysis, called consolidated bioprocessing (CBP). CBP involves four biologically-mediated events: (1) enzyme production, (2) substrate hydrolysis, (3) hexose fermentation and (4) pentose fermentation. In contrast to conventional approaches, with each step performed independently, all four events may be performed simultaneously in a CBP configuration. This strategy requires a microorganism that utilizes both cellulose and hemicellulose. A CBP process that utilizes more than one organism to accomplish the four biologically-mediated events is referred to as a consolidated bioprocessing co-culture fermentation. Currently there is a lack of a fermentation microorganism that can effectively hydrolyze cellulose and hemicellulose as well as convert all biomass sugars, especially xylose and arabinose as well as glucose, to final products.
An ideal CBP microorganism should be able to produce ethanol as sole product, hydrolyze cellulose to fermentable oligomers, hydrolyze hemicellulose to fermentable oligomers, ferment cellulose oligomers, ferment xylose or xylose oligomers, produce ethanol in high titer (resistant to up to 45% ethanol), be resistant to up to 1% acetic acid from hemicelluloses, grow at thermophilic temperatures ranging from 55 to 80° C., be moderately resistant to common pretreatment inhibitors (furans, polyphenolics) and produce a multi-carbohydrase portfolio on the cellulosome [Mielenz (2009) in Molecular Biology and Biotechnology, 5th Edition, Ed. J. M. Walker & R. Rapley, Royal Society of Chemistry, pp: 548-584]. No such single microorganism is presently known and the present invention addresses this need by providing two groups of microorganisms which have together satisfy many of these characteristics, and when co-cultured, can efficiently achieve CBP.